The present invention relates to a tire for motor vehicle wheels having a tread producing a low rolling noise, which comprises a raised pattern formed of a plurality of shaped blocks distributed in parallel circumferential rows bounded by longitudinal grooves extending circumferentially around the tire with each row comprising a given number of shaped blocks separated from each other by respective transverse generally axially expanding cuts.
It is known that tires for road motor vehicles have a plurality of longitudinal grooves and generally axially extending transverse cuts on the tread, which define a plurality of shaped blocks thereon, said blocks being distributed according to a carefully determined specific pattern.
Within the tread pattern in which the longitudinal grooves are positioned the shaped blocks are distributed in several side by side rows. A proper distribution gives the tire the desired features of directional control stability and road holding in relation to the sidewise thrusts directed parallel to the wheel axis. The transverse cuts, in turn, give the tire the desired road traction, that is the capability of efficiently transmitting tangential thrusts parallel to the running direction during speeding up and slowing down of the vehicle. In principle, the traction of the tire tends to become greater as the number of the transverse cuts arranged in the tread increases and the orientation of the cuts themselves approach an orientation which is perpendicular to the running direction.
In addition, the longitudinal grooves and transverse cuts cooperate in performing an efficient draining action of water from the tread pattern area of the tire during running on a wet road bed.
The presence of the transverse cuts and longitudinal grooves however causes the well known effect of noise produced by the rolling tire. One of the main causes of rolling noise has been found to be caused by the continuous succession of impacts of the shaped block corners on the road bed.
Another contributory cause in generating noise is the rubbing that the shaped blocks undergo in contact with the road bed when they enter and leave the ground contact patch. These rubbings are essentially due to the deformations necessarily undergone by the tread, when its outer surface, which in a free state has a convex conformation, inevitably becomes flat against the road bed.
The dynamic actions causing the deformation of the tread between the entry and exit regions to and from the contact patch also give rise to a cyclic variation of the volume of the grooves and cuts defined between the shaped blocks, which brings about a cyclic compressive and decompressive action on the air contained in the grooves and cuts. This phenomena of air compression and decompression assist in the generation of noise produced by the rolling tire.
The known art suggests several expedients for limiting as much as possible the noise produced by a tire rolling.
One expedient which has been adopted for a long time essentially consists of differentiating the longitudinal dimensions, that is those along the circumferential extension, of the single shaped blocks. In other words, the blocks are disposed on the circumferential extension of the tread according to two or more differentiated pitch values distributed in a circumferential succession, usually referred to as "pitch sequence" which is generally carefully laid out so as to achieve the maximum possible avoidance of pattern repetition around the tire circumference. In this manner the pressure waves generated by the impacts and rubbings of the blocks do not take place according to a specific frequency on which most of the acoustic energy generated as a result of rolling would be concentrated. In other words, the acoustic energy is distributed over a wide frequency range, thereby eliminating the troublesome effect of harmonic repetition and noise usually known as "squirrel effect".
For the purpose of distributing the acoustic energy over a greater number of frequencies, it has also been suggested to carry out offsetting, with respect to each other, of the blocks belonging to circumferential rows disposed consecutively in side by side relation. This solution however makes it difficult to drain the water from the center of the contact area towards the outer edges of the tread and therefore makes it necessary to increase the width of the longitudinal grooves for achieving a sufficient draining action.
Another expedient consists in giving the transverse cuts a suitably inclined orientation with respect to tire's axis of rotation. This enables the impacts of the block corners on the road bed to be "smoothed" but, on the other hand, it impairs the road traction of the tire.
A considerable reduction of the noise produced by rolling has been achieved by forming additional narrow cuts usually referred to as "sipes" in the shaped blocks, said sipes acting to increase the elastic deformability of the blocks. This solution too enables the impact of the block on the road bed to be smoothed, thereby causing a reduction of the noise resulting from impacts. However a greater deformability of a block will tend to increase the noise effects resulting from rubbings.
By way of example of the above described state of the art, reference is made to Assignee's prior U.S. Pat. No. 5,287,905 the disclosure of which is hereby incorporated by reference.